The user database is built from a text file
using the
makemap utility
(in the distribution in the makemap subdirectory).
The text file is a series of lines corresponding to userdb records;
each line has a key and a value separated by white space.
The key is always in the format described above --
for example:

eric:maildrop

This file is normally installed in a system directory;
for example, it might be called
/etc/userdb. To make the database version of the map, run the program:

makemap btree /etc/userdb.db < /etc/userdb

Then create a config file that uses this.
For example, using the V8 M4 configuration, include the
following line in your .mc file: